I really don't know what to do this year...
Hi. I get your predicament. It's not fun to be the peacemaker.
Here's my suggestion about Thanksgiving: Invite your STBX to eat the traditional meal with you and the kids. Invite MIL, too. Just invest in the day, don't try to make it represent everything you wish for. It will be a great way to spend the day as a family for your kids and honor your MIL whom you obviously like.
THEN, my suggestion is you get your STBX one-on-one ASAP and just have an honest talk. You need to know is he really interested in working on the marriage or not, period. Not, "well, um,....." It has to be "yes" or "no." And if it's "yes" it needs to come with joint marriage counseling, NO DATING on either of your parts, and a regular visitation schedule for your children. You BOTH need to commit to making it work. If you don't, then it's for naught. If it's "no" then you need to decide whether your filing the divorce papers or he is. If he's again hemming and hawing, well, it's you filing. Just do it. Someone has to actually DO SOMETHING to get out of limbo. You can't stay in limbo for long.
I think maybe inviting your stbx would be okay but if you can I would also invite maybe one or two other people..this way you could start a new tradition.. and have some new people around. Its a whole new start and by taking baby steps this could be just one baby step to being without your ex. When I got divorced I didnt know where I was going to spend the holidays.. Well? Me and my sis started a new holiday tradition.. We have people over and everyone brings something. Every year we have added new people and we have a blast.. I can barely remember how I celebrated things in the past because I have started new beginnings or maybe different beginnings.. but its all working out for the better.
I agree with wisdom on the counseling and all but I would also start doing a few things alone .. Find a balance between working on the marriage and finding things for yourself to do. Maybe taking a dance class or take up a hobby or take